The present invention is related to the field of pressure transducers, and more particularly to capacitive pressure transducers in which sensed pressure causes a predetermined capacitance by altering the spacings between electrodes of a variable capacitor having at least one electrode on a pressure displaceable diaphragm. The capacitance is then utilized to provide an electrical signal representative of the sensed pressure.
Capacitive pressure transducers are known which comprise an etched portion of silicon acting as a pressure sensing diaphragm and also as one electrode of the capacitive pressure transducer. Typically, the other electrode of the capacitive transducer comprises metallization deposited on an insulating base substrate which is many times thicker than the etched silicon diaphragm. U.S. Pat. No. 4,384,899 to Myers, and assigned to the same assignee as the present invention, illustrates such a structure and describes how anodic bonding techniques can be beneficially utilized to bond the insulating base substrate to the silicon diaphragm.
In manufacturing the pressure transducers described in the above noted Myers patent, the capacitive electrode on the base substrate comprises a film metallization which is deposited on an external surface of the base substrate to form a desired base electrode pattern. Subsequently the silicon diaphragm is bonded to the base substrate with a recessed portion of the silicon diaphragm spaced apart from and facing the film metallization layer on the base substrate. This results in effectively surrounding the base substrate electrode by the silicon diaphragm. Since electrical contact must be made to the base substrate capacitive electrode, typically film metallization is deposited in a through hole in the base substrate thereby providing an accessible external electrical connection to the base substrate electrode (comprising the film metallization on the external surface of the base substrate).
The above noted Myers patent discusses in detail a beneficial anodic bonding technique which advantageously bonds the silicon diaphragm to nonconductive peripheral portions of the base external surface which surround the base film metallization that forms one of the capacitor electrodes. The Myers structure illustrates the use of film metallization for the base electrode, and also for providing a feedthrough connection through the base substrate. Providing these film metallizations is costly since they require complex manufacturing processes and costly and complex equipment. Typically the film metallization is provided by a thin film evaporation or RF sputtering technique. This technique also involves either utilization of an appropriate mask or utilization of photographic coat, exposure, develop and etching techniques to define the pattern of the film metallization forming the base electrode. In addition, depositing film metallization in the through hole of the base substrate to provide a feedthrough connection for providing external access to the base electrode has proved to be somewhat difficult to reliably implement.
In addition to the above noted deficiencies of the prior capacitive transducers, accuracy of the film metallization electrode pattern must be precisely controlled, since this will determine the nominal capacitance provided by the pressure transducer. Only pressure transducers which fall within a narrow range of nominal capacitance would be useable in most applications. Also, typically it is desired to provide a nominal capacitance for the pressure transducer which is as large as possible. This results in having the recessed portion of the silicon diaphragm spaced very close to the film metallization comprising the base substrate electrode. Therefore even minor variations in the thickness of this film metallization could substantially effect the nominal capacitance of the pressure sensor. The anodic bonding technique described in the above noted Myers patent was developed to minimize the problem of variation of nominal capacitance, as well as other problems, caused by the previous utilization of an interface bonding material between the base substrate and the silicon diaphragm. However, still precise control must be used in the film metallization process to ensure proper nominal capacitance and to minimize feedthrough reliability problems. This adds to the cost of such prior transducers.